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Chapter 31 Alternating Current PowerPoint® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson Copyright © 2012 Pearson Education Inc. Goals for Chapter 31 • To use phasors to describe sinusoidally varying quantities • To use reactance to describe voltage in a circuit • To analyze an L-R-C series circuit • To determine power in ac circuits • To see how an L-R-C circuit responds to frequency • To learn how transformers work Copyright © 2012 Pearson Education Inc. Introduction • How does a radio tune to a particular station? • How are ac circuits different from dc circuits? • We shall see how resistors, capacitors, and inductors behave with a sinusoidally varying voltage source. Copyright © 2012 Pearson Education Inc. Phasors and alternating currents • Follow the text discussion of alternating current and phasors using Figures 31.1 (which shows ac voltage) and 31.2 (which shows a phasor diagram) below. Copyright © 2012 Pearson Education Inc. Root-mean-square values • Follow the text discussion of rectified alternating current, rms current, and rms voltage. Use Figures 31.3 (right) and 31.4 (below). Copyright © 2012 Pearson Education Inc. Current in a personal computer The plate on a personal computer says that it draws 2.7 A from a 120-V 60 Hz line. a)What is the average current? Copyright © 2012 Pearson Education Inc. Resistors in an ac circuit • Ohm’s Law gives the voltage amplitude across a resistor: VR = IR. • Figure 31.7 shows the circuit, the current and voltage as functions of time, and a phasor. Copyright © 2012 Pearson Education Inc. Inductors in an ac circuit • The voltage amplitude across the inductor is VL = IXL. • Example 31.2. Voltage leads current in an inductor ELI Copyright © 2012 Pearson Education Inc. Capacitance in an ac circuit • The voltage amplitude across the capacitor is VC = IXC. ICE Copyright © 2012 Pearson Education Inc. A resistor and a capacitor in an ac circuit • Example 31.3, a resistor and a capacitor in an ac circuit. Copyright © 2012 Pearson Education Inc. Comparing ac circuit elements • Table 31.1 summarizes the characteristics of a resistor, an inductor, and a capacitor in an ac circuit. ELI the ICE man Copyright © 2012 Pearson Education Inc. Comparing ac circuit elements • Figure 31.11 (below) shows graphs of resistance and reactance as a function of ω. What equations are responsible for these curves? Copyright © 2012 Pearson Education Inc. A useful application: the loudspeaker • The woofer (low tones) and the tweeter (high tones) are connected in parallel across the amplifier output. Copyright © 2012 Pearson Education Inc. The L-R-C series circuit • Using superposition, we can understand what happens when all three components are combined, using Figure 31.13 below. • The voltage amplitude across an ac circuit is V = IZ. • Example 31.4 Copyright © 2012 Pearson Education Inc. Power in ac circuits • Follow the text discussion of power in alternating-current circuits using Figure 31.16 below. • Note that the net energy transfer over one cycle is zero for an inductor and a capacitor. • Example 31.7. Copyright © 2012 Pearson Education Inc. Resonance in ac circuits • At the resonance angular frequency 0, the inductive reactance equals the capacitive reactance and the current amplitude is greatest. (See Figure 31.18 below.) Copyright © 2012 Pearson Education Inc. Tuning a radio • Follow Example 31.8 using Figure 31.20 below. Copyright © 2012 Pearson Education Inc. Transformers • Power is supplied to the primary and delivered from the secondary. See Figure 31.21 at the right. • Terminal voltages: V2/V1 = N2/N1. • Currents in primary and secondary: V1I1 = V2I2. Copyright © 2012 Pearson Education Inc. Real transformers • Real transformers always have some power losses, as illustrated in Figure 31.24 below. • Follow Example 31.9. Copyright © 2012 Pearson Education Inc.